4 ms·

Why do you keep denying the principle when people have already pointed out that solar updraft towers (which exploit the exact same principle) exist and work? ht

by mrb 9y ago

Why do you keep denying the principle when people have already pointed out that solar updraft towers (which exploit the exact same principle) exist and work? https://en.m.wikipedia.org/wiki/Solar_updraft_tower https://en.m.wikipedia.org/wiki/Solar_updraft_tower

fenomas 9y ago
That link doesn't support the article's thesis at all. It describes a system where the low-lying air has been heated up to be significantly warmer than surrounding atmospheric air. The energy being extracted is coming from solar power, not from the temperature difference at different levels of the atmosphere.
gus_massa 9y ago
From that article: > The chimney had a height of 195 metres (640 ft) and a diameter of 10 metres (33 ft) with a collection area (greenhouse) of 46 hectares (110 acres) and a diameter of 244 metres (801 ft), obtaining a maximum power output of about 50 kW. That's not a super chimney. It's a normal chimney. People believe in normal chimneys! You have a temperature difference between the bottom and the top of the chimney, but it is so short tat you can assume that the pressure of the surrounding air is constant, and that the pressure of the air in the chimney is constant. When the chimney is very high, the pressure inside and outside the chimney changes with the height, so the temperature inside and outside changes with the height. So to compute the difference in the temperature you have to adjust it. They are implicitly using that the temperature changes outside and ignoring that it will change inside too. Also, the design/experiments in Wikipedia use a large glass bell or something similar to trap the solar heated air and send it to the chimney. So they are replacing a good fire of firewood with solar power. But they ignore the HUGE glass bell in the project and in the video. They expect that the hot air will go spontaneously into the chimney instead of trying to go up in another path. Trying to harvest some of the energy and also using the air column to keep the column up will make the hot air in the surface prefer to go in another path and ignore the chimney. Unless you put a huge glass bell to force the air path. By the way, the design/experiments in Wikipedia use self porting towers, they don't use the same air to keep the structure up. I think that the idea of using the flow to keep the structure up is very fishy, but my handwaving is not powerful enough to be sure it's wrong, I'd like to see some calculations. But you surely need the glass bell too.
- mikeash 9y ago
  "They are implicitly using that the temperature changes outside and ignoring that it will change inside too." I just want to point out that they are quite explicit about this. That's what prompted my original comment, and I quoted it there. That's why I can't figure out why it's not obvious to everyone that it's full of crap. They all but come right out and say, "This idea is full of crap because it's based on a terrible assumption which you can see right here."
- mrb 9y ago
  Imagine a normal 200m solar updraft chimney at elevation 0km; it sustains enough updraft to work, and spans 0 to 0.2km. Now imagine another 200m chimney at elevation 0.2km; it too is self-sustaining and spans 0.2km to 0.4km. If we stacked these 2 chimneys on top of each other, it would be a self-sustaining 400m chimney. Repeat this for 5km, and it's a self-sustaining super chimney. But according to you this physical mechanism somehow breaks between 200m and 5km... The glass bell may be necessary for a 5km chimney but I don't see why this would invalidate the physical principle behind it.
  - mikeash 9y ago
    It's not self sustaining, it's sustained by the sun. When the sun goes down, the chimney quits. For a 200m chimney that supports itself physically, that's fine. For a 5km chimney that relies on continuous airflow to hold itself up, that's substantially less fine.
    - mrb 9y ago
      "it's sustained by the sun." That's what I meant. But you fail to explain why, if the principle works for a series of 200m chimneys at different elevations, it would somehow stop working if these chimneys were connected/stacked on top of each other. I'm not arguing about the engineering difficulty of building a self-supporting chimney made of fabric, or whether or not it works at night. Just the thermodynamic aspect of it even working during the day.
      - 9y ago
mikeash 9y ago
Because "the principle" involves operating 24/7 to support itself based on the temperature difference between the top and bottom, and handwaving that the air inside will somehow magically not expand and cool like it would outside. Obviously solar updraft towers work, but they exploit the temperature difference between air at the base of the chimney and air outside the chimney. Completely different.

by mrb 9y ago

fenomas 9y ago

That link doesn't support the article's thesis at all. It describes a system where the low-lying air has been heated up to be significantly warmer than surrounding atmospheric air. The energy being extracted is coming from solar power, not from the temperature difference at different levels of the atmosphere.

gus_massa 9y ago

From that article: > The chimney had a height of 195 metres (640 ft) and a diameter of 10 metres (33 ft) with a collection area (greenhouse) of 46 hectares (110 acres) and a diameter of 244 metres (801 ft), obtaining a maximum power output of about 50 kW. That's not a super chimney. It's a normal chimney. People believe in normal chimneys! You have a temperature difference between the bottom and the top of the chimney, but it is so short tat you can assume that the pressure of the surrounding air is constant, and that the pressure of the air in the chimney is constant. When the chimney is very high, the pressure inside and outside the chimney changes with the height, so the temperature inside and outside changes with the height. So to compute the difference in the temperature you have to adjust it. They are implicitly using that the temperature changes outside and ignoring that it will change inside too. Also, the design/experiments in Wikipedia use a large glass bell or something similar to trap the solar heated air and send it to the chimney. So they are replacing a good fire of firewood with solar power. But they ignore the HUGE glass bell in the project and in the video. They expect that the hot air will go spontaneously into the chimney instead of trying to go up in another path. Trying to harvest some of the energy and also using the air column to keep the column up will make the hot air in the surface prefer to go in another path and ignore the chimney. Unless you put a huge glass bell to force the air path. By the way, the design/experiments in Wikipedia use self porting towers, they don't use the same air to keep the structure up. I think that the idea of using the flow to keep the structure up is very fishy, but my handwaving is not powerful enough to be sure it's wrong, I'd like to see some calculations. But you surely need the glass bell too.

mikeash 9y ago
"They are implicitly using that the temperature changes outside and ignoring that it will change inside too." I just want to point out that they are quite explicit about this. That's what prompted my original comment, and I quoted it there. That's why I can't figure out why it's not obvious to everyone that it's full of crap. They all but come right out and say, "This idea is full of crap because it's based on a terrible assumption which you can see right here."
mrb 9y ago
Imagine a normal 200m solar updraft chimney at elevation 0km; it sustains enough updraft to work, and spans 0 to 0.2km. Now imagine another 200m chimney at elevation 0.2km; it too is self-sustaining and spans 0.2km to 0.4km. If we stacked these 2 chimneys on top of each other, it would be a self-sustaining 400m chimney. Repeat this for 5km, and it's a self-sustaining super chimney. But according to you this physical mechanism somehow breaks between 200m and 5km... The glass bell may be necessary for a 5km chimney but I don't see why this would invalidate the physical principle behind it.
- mikeash 9y ago
  It's not self sustaining, it's sustained by the sun. When the sun goes down, the chimney quits. For a 200m chimney that supports itself physically, that's fine. For a 5km chimney that relies on continuous airflow to hold itself up, that's substantially less fine.
  - mrb 9y ago
    "it's sustained by the sun." That's what I meant. But you fail to explain why, if the principle works for a series of 200m chimneys at different elevations, it would somehow stop working if these chimneys were connected/stacked on top of each other. I'm not arguing about the engineering difficulty of building a self-supporting chimney made of fabric, or whether or not it works at night. Just the thermodynamic aspect of it even working during the day.
    - 9y ago

mikeash 9y ago

Because "the principle" involves operating 24/7 to support itself based on the temperature difference between the top and bottom, and handwaving that the air inside will somehow magically not expand and cool like it would outside. Obviously solar updraft towers work, but they exploit the temperature difference between air at the base of the chimney and air outside the chimney. Completely different.